Gas turbine engines are typically employed to power aircraft. Typically a gas turbine engine will comprise an axial fan driven by an engine core. The engine core is generally made up of one or more turbines which drive respective compressors via coaxial shafts. The fan is usually driven off an additional lower pressure turbine in the engine core.
The fan comprises an array of radially extending fan blades mounted on a rotor. The fan blades and/or a casing that surrounds the fan may be manufactured from metallic and/or composite laminate materials. Generally fan blades include a composite body and may be provided with a metallic leading edge and/or a metallic trailing edge.
Composite components are often laminate structures that include a plurality of plies. Each ply generally includes reinforcing fibres (e.g. high strength or high stiffness fibres) embedded in a matrix, e.g. a plastic matrix material. The matrix material of adjacent stacked plies is bonded together to build the composite component. The matrix material is weaker than the fibre material and as such the bond between stacked plies can form a point of weakness. This means that a primary failure mechanism of concern for composite materials is delamination.
Delamination for example of a fan blade may occur in the event of an impact by a foreign object such as a bird strike.
To reduce the risk of delamination of a composite component through-thickness reinforcement can be used. One type of through-thickness reinforcement is pinning (which may also be referred to as Z-pinning). A component that has been pinned includes a plurality of pins (which may be referred to as Z-pins or rods) extending through the thickness of the component in a direction transverse to the general direction of the plies. The pins are generally made of a metallic or composite material and typically have a diameter ranging from or equal to approximately 0.3 mm to 0.5 mm. Often, composite pins are manufactured by pultrusion of a carbon fibre tow impregnated by a thermoset resin. The pins of a composite component exert a bridging force on the plies to hold the plies in position relative to each other, this suppresses opening of inter-laminar cracks (known as mode I failure) and sliding displacements of inter-laminar cracks (known as mode II failure).
Although the pins used in through-thickness reinforcement can substantially arrest crack propagation, it is often not possible to eliminate crack propagation in a composite component. Accordingly, it would be useful to detect when delamination has occurred, and the extent and/or location of such delamination so as to indicate when a component needs repair or replacement.